Secretion of pepsinogen by the stomach is an important component of the digestive process. In addition to initiating protein digestion, pepsins release peptides and amino acids which serve as stimuli for release of the regulatory hormones gastrin and CCK. Moreover, the presence of pepsins in gastric juice is believed to enhance the effect of acid in the formation of gastric and duodenal ulcers. The overall goal of this research program is to characterize the physiological mechanisms which regulate pepsinogen secretion. The proposed research will take advantage of the development of isolated gastric glands as a model system for the in vitro study of pepsinogen secretion. There are three major aims of the present proposal. The first aim is to identify the role of peptide hormones in stimulating pepsinogen secretion. These studies will focus on the question of whether gastrin is a direct physiological stimulus for pepsinogen secretion as is generally believed. Radioligand binding will be used to characterize peptide receptors and identify their cellular location. In addition, attempts will be made to identify a peptide stimulus found in partially purified duodenal extracts. These studies propose to fractionate the crude preparation using HPLC and identify active peptides through compositional analysis. The second major aim of the proposal seeks to understand the physiological significance of the existence of two separable intracellular mechanisms leading to pepsinogen secretion. This question will be approached using ligand binding to define the cellular distribution of receptors associated with the two pathways and by characterizing the pattern of stimulation elicited by the two mechanisms. The final aim of the proposal will employ a novel preparation of permeabilized gastric glands to investigate the intracellular requirements for pepsinogen release. Emphasis will be placed on examining the roles of ionic composition, pH and second messengers as possible candidates for regulating peptic granule release.